Actually, they aren't claiming it's a choice. They are claiming that people's brains are naturally wired certain ways (often male or female) that does not match their outter bodies.
This would explain those who identify as male, female, and neutral, but doesn't explain all the other nonsense identities they want to tie in with "gender".
It also does not negate that their outer bodies are their outer bodies. So a male body does not belong in women's sports.